1. Field of the Invention
The present invention relates to shaped yarns and industrial fabrics. More specifically, the present invention relates to using yarns with longitudinally oriented grooves to reduce fabric permeability without the need of an additional coating or stuffer yarns. These yarns can also be bicomponent yarns with improved coating adhesion, or bicomponent yarns with a wear-indicating mechanism. The invention also relates to grooved yarns that exhibit increased coating adhesion whilst providing improved sheet and air handling in fabrics.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Contemporary papermaking fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of the synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back-and-forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing at least one base fabric within the endless loop formed by another, and by needling a staple fiber batt through these base fabrics to join them to one another. One or more of these woven base fabrics may be of the on-machine-seamable type. This is now a well known laminated press fabric with a multiple base support structure.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross.
Turning now to the yarns used heretofore, particularly for dryer fabrics, monofilament yarns have typically been extruded with a simple circular cross-section. More recently, monofilaments with shaped cross-section have been produced. These shaped monofilaments have been used in woven fabrics to modify the fabric surface texture or density, or in particular, to control the fabric air permeability. In this connection, for example, U.S. Pat. No. 5,361,808 (Bowen) discloses using finned or T-shaped monofilaments as CD stuffer yarns to reduce air permeability. As another example, U.S. Pat. No. 5,998,310 (Bowen) shows a tri-lobal stuffer used to reduce permeability. “Y” and “X” and “T” shaped monofilaments are also described. Fabric stability at permeabilities of 200 CFM or greater using the shaped cross-machine-direction (CD) yarns is maintained. None of the prior art however, uses shaped yarns as functional yarns which reduce air permeability without using a coating and without using stuffer yarns. Nor does any of the prior art use shaped CD monofilaments for improved coating adhesion and for producing bicomponent monofilaments.
Also in connection with round MD yarns used heretofore in dryer fabrics, the fabric's sheet side has been treated with a coating that grips the paper sheet. Although the coating has sufficient abrasion resistance, there is concern that the coating will detach from the circular yarns prematurely. In addition, the fluoropolymer in the yarn and oils on its surface impeded attachment of the coating to the monofilaments. Adequate venting of moisture at the point of contact with the paper sheet, and sufficient air handling along the fabric surface have also been concerns.